This invention relates in general to pumps for supplying pressurized fluid to hydraulically actuated devices. In particular, this invention relates to a continuously driven gear pump having an internal bypass valve for directing the flow of pressurized fluid from its outlet port either to the actuated device (when the gear pump is operated in the active mode) or back to its inlet port (when the gear pump is operated in the bypass mode), thereby selectively controlling the operation of the device. This invention also relates to a means for cooling the gear pump when it is operated in the bypass mode.
Gear pumps are well known structures which typically include a pair of gears mounted upon respective shafts for rotation within a pump housing. The shafts are arranged such that the gears mesh within a pumping chamber disposed between an inlet port and an outlet port of the gear pump. One of the shafts is rotated by an external source of power so as to cause the two gears to rotate. The cooperation of the gears with the pumping chamber causes fluid to be drawn from a reservoir through the inlet port and be discharged from the outlet port to the hydraulically actuated device. The device is responsive to the pressurized fluid from the gear pump for being actuated in a known manner.
One common use for gear pumps is on refuse collection vehicles having one or more hydraulically actuating packing devices. The gear pump is usually connected to and driven by the internal combustion engine of the vehicle so as to selectively generate a flow of pressurized fluid to operate the packing devices. Because of its size and reliability, the gear pump is well suited to perform this function. Typically, however, such packing devices are used only intermittently, requiring no flow of pressurized fluid for long periods of time, while the internal combustion engine is usually continuously operated. Thus, for this and other gear pump applications, some means must be provided for selectively interrupting the flow of pressurized fluid from the gear pump to the hydraulic device.
Several structures are known in the art for accomplishing this selective interruption of fluid flow from the gear pump to the actuated device. A first known structure includes an external bypass valve connected between the outlet port of the gear pump (which is continuously driven by the engine) and the actuated device. The external bypass valve is controlled by an operator to selectively direct the flow of pressurized fluid from the outlet port of the gear pump either to the device or back to the reservoir. Thus, when the device is not to be operated, the flow of pressurized fluid is directed from the outlet port of the gear pump back to the reservoir, bypassing the device. Unfortunately, this structure results in undesirable power losses, particularly at high engine speeds, because fluid is continuously pumped a relatively long distance from the reservoir to the pump and back to the reservoir.
A second known structure includes a power take-off unit or a clutch connected between the engine and the gear pump. The power take-off unit or clutch selectively makes and breaks the rotational driving connection between the internal combustion engine and the gear pump. When the device is not to be operated, the power take-off or clutch is disengaged so as to disable the gear pump. As a result, the flow of pressurized fluid to the device is interrupted. While such structures are efficient because they do not impose any load on the engine when the device is not in use, they are typically expensive, complicated, and prone to failure.
A third known structure includes a dry valve, which can simply be described as a shut-off valve disposed in the inlet port of the gear pump. When closed, the dry valve obstructs the flow of fluid from the inlet port into the pumping chamber of the gear pump. Consequently, the flow of pressurized fluid to the device is interrupted, even though the gear pump is continuously driven by the engine. Typically, means are provided in the dry valve for permitting a relatively small amount of fluid to flow into the pumping chamber even when the dry valve is closed. Such relatively small amount of fluid flow is necessary for lubricating and cooling the components of the gear pump while it is operated in the dry mode. However, means must also be provided for preventing this small amount of fluid from being pumped to the device and inadvertently actuating same when the gear pump is operated in the dry mode. Such means can include a flow control valve disposed in the outlet port of the pump for diverting the small amount of fluid back to the reservoir.